When communicating over a communication network, it is often helpful to be able to characterize the communication medium over which the communication is being transmitted. As used herein, “channel” refers to the particular medium (for example, wireless, coaxial cable, copper twisted pair or fiber optic cable) used to communicate between a transmitter and at least one receiver, the frequency used, as well as the modulation or other means for encoding information communicated over the channel. The process of characterizing the channel is commonly referred to as “channel estimation.” Channel estimation may be performed in several ways.
One approach for channel estimation is through the use of a preamble which is sent immediately ahead of a portion of the information to be sent. That is, content of a message is typically divided into relatively small portions and sent in separate packets. Each packet includes several components, including the payload which contains the information that the transmitter is attempting to send to the receivers. Some of the other components in a packet include a preamble and a forward error correction (FEC) field. As is well known, bursty transmissions of packets allow transmission of small amounts of data over different channels to be interleaved. That is, a first packet of information can be transmitted to a first receiving node in a network from a transmitting node over a first channel. Immediately after the transmission of this first packet a second packet can be transmitted from either the first node or a second node to either the same recipients or to a different one or more receiving nodes. This differs from continuous transmissions in which a channel remains in use for an extended period of time.
Orthogonal frequency division multiplexing (OFDM) or orthogonal frequency division multiple access (OFDMA) transmissions typically involve the use of a preamble. In OFDM a signal is transmitted using multiple subcarriers at distinct frequencies. The subcarriers are received and assembled at the receiver to enable high-speed communication. OFDMA is a type of OFDM transmission in which dynamic assignment of a subset of the subcarriers to individual users allows multiple users to transmit at the same time. The signals of several different users will each be assigned to be transmitted on one or more unique subcarriers. Each subcarrier is generated and transmitted in a manner that allows subcarriers from different users to be transmitted concurrently without interfering with one another, enabling multiple access. Therefore, independent information streams can be modulated onto each subcarrier whereby each such subcarrier can carry independent information from a transmitter to one or more receivers.
Typically, unlike transmissions in a continuous transmission system, transmissions in a bursty packet based OFDM and OFDMA transmission system require a full preamble for each packet transmitted on each subcarrier in a time-frequency grant. For the purpose of the present discussion, a “full preamble” contains enough reference signal information for the receiver to determine a total channel estimate for every subcarrier to be used in the transmission. In some cases, the preamble is used to characterize the channel (i.e., perform channel estimation) by carrying a known set of information that can be interpreted at the receiver to determine any distortions that have occurred as a consequence of the characteristics of the channel. In other words, use of a preamble enables the receiver to compare an ideal channel against a channel that has distortion. In this way, the receiver can estimate the total distortion on that channel. That estimate includes measuring the combined effect of the channel plus the effect of any offset of the transmitter's carrier phase (i.e., any offset in the phase of the carrier used by the transmitter relative to the phase of the receiver's carrier). Such transmitter phase offset might be due to drift in an oscillator that generates the carrier frequency within the transmitter with respect to the oscillator within an intended receiver. The drift that can cause such an offset is typically constrained by a communications protocol specification that dictates the performance and operation of the network. One typical constraint on the drift may be 150 Hz/msec or 300 Hz/sec. Such drift can occur even if the upstream transmitter is phase-locked to the downstream broadcast from a receiver. In addition, the receiver uses the channel estimate to calculate inverse equalization factors.
Any time a channel is occupied by a preamble, the preamble reduces the channel capacity (amount of payload information that can be transmitted via the channel). Subcarriers used for transmitting preamble data at particular times are not available to transmit payload data at those times. As a result, the preamble constitutes overhead (in time-frequency) that taxes system resources. Therefore, it is generally advantageous to reduce the amount of overhead attributed to the preamble.